54 HOMOLOGIES OF A LOBSTER. 



said, that which is homologous with the belly of 

 vertebrate animals. 



To suit the inverted position of arthropodous 

 animals, the mouth has been placed at what corre- 

 sponds to the back of the head in vertebrate animals, 

 and in order to permit the necessary communication 

 between it and the stomach, which retains its relative 

 position, the oesophagus or gullet passes through the 

 aesophageal nervous collar, which may be viewed as 

 the medulla oblongata of vertebrate animals split 

 into lateral halves by a longitudinal fissure a mere 

 mechanical adaptation like the passage of a tendon 

 through a split in another without involving any 

 real fundamental difference of structure or function. 



The central organ of the nervous system in 

 arthropoda the so-called ventral chain of ganglions, 

 but really and absolutely the homologue of the spinal 

 cord of vertebrata, was shown in 1832 by my much 

 respected colleague, the late Dr. Grant, to be com- 

 posed of two columns the one ganglionic, the other 

 non-ganglionic. But in accordance with the mistaken 

 view as to the real dorsal aspect of the lobster or 

 insect's body, it was supposed that the relative position 

 of the two columns the ganglionic or sensitive 

 ' below,' and the non-ganglionic or motor ' above ' 

 was the reverse of what it is in vertebrate animals. 

 From my demonstration, however, of the true dorsal 

 aspect of arthropoda, it is evident that the relative 

 position of the two columns corresponds exactly with 



